The present invention relates to a disk-like information recording medium for recording various information signals such as an audio signal and a video signal.
As media for recording various kinds of information such as audio information and video information, there are known disk-like optical recording media or magnetic recording media, examples of which include a phase change type optical disk in which information signals are written in the form of irregularities, typically embossed pits or grooves, a magneto-optical disk making use of a magneto-optical effect of a recording film, and a magnetic disk.
A disk substrate having fine irregularities, typically prepits or grooves, is obtained by injection molding a plastic material.
The major drawback of a plastic substrate molded by injection molding lies in that stress is induced by an effect of friction of molten resin against a mold when the molten resin is injected and charged in a cavity of the mold and by effects of pressure and temperature applied to the molten resin upon injection of the molten resin, and the stress remains in the finished substrate. The inner stress of the substrate becomes somewhat small by stress relief taking place during a period of time from cooling to solidification; however, most of the inner stress is not relieved until solidification and remains as a residual stress in the substrate. Such a residual stress may cause birefringence of light and/or warping or waviness of the substrate.
The occurrence stages of the inner stress of a substrate is as follows:
(1) upon injection of resin; shear stress or strain due to molecular orientation occurs in the resin when the resin is fluidized in a cavity; PA1 (2) upon completion of filling; the motion of a screw is rapidly stopped and also the fluidization of the resin is rapidly stopped when the cavity is filled with the resin, so that inertia, forces of the resin and the screw are all applied to the substrate; PA1 (3) at a pressure retention stage; since a pressure is applied to the resin for preventing counter-flow of the resin and preventing shrinkage due to contraction of volume until the resin having been injected is sealed with a gate, a pressure distribution occurs over the entire substrate; and PA1 (4) upon cooling the substrate; stress depending on a temperature distribution occurs in the substrate by thermal contraction. PA1 (1) contraction of outer peripheral portion&lt;contraction of inner peripheral portion=deformation in the form of a propeller; and PA1 (2) contraction of outer peripheral portion&gt;contraction of inner peripheral portion=deformation in the form of a bowl.
A plastic substrate for an optical information medium or a magnetic recording medium is necessarily contracted by molding. In many cases, the molding contraction of an outer peripheral portion of the plastic substrate is different from that of an inner peripheral portion of the plastic substrate. That is to say, the molding contraction of the outer and inner peripheral portions occurs as follows:
In order to reduce both the deformation due to molding contraction and the deformation due to residual stress induced upon injection of the resin, according to a related art molding method, an attempt has been generally made to reduce the inner pressure applied to the resin by weakening a clamping pressure, retarding the injection speed, and reducing the amount of the resin to fill the cavity. Such an attempt, however, presents another problem that shrinkage or depression and/or swelling occur at an outer peripheral portion of a disk, tending to cause a failure in transfer of information signals and to degrade signal characteristics.
In addition, since the strength of a disk is proportional to the cube of the thickness of the disk, mechanical characteristics such as a flexural strength of the disk become poor as the thickness of the disk becomes thinner with an increase in recording density, and bimetal deformation may significantly occur by heat and film stress caused upon film formation and by moisture absorption in a temperature/moisture environment, particularly, for an optical recording medium of a one-sided recording type.
To solve the above problem, it is desirable to adopt a material having high stiffness, that is, a high flexural or tensile Young's modulus; however, a single plastic material other than glass or metal cannot satisfy such a high stiffness, and a composite material of a plastic material to which fibers or fillers are added can satisfy the above high stiffness but presents problems that surface portions of the fillers or fibers are exposed from the surface of the substrate, to cause surface roughness of the substrate, failing to ensure micro-flatness of the surface of the substrate excluding transferred fine irregularities, and accordingly, such a composite material cannot be applied to a substrate, for example, used for a disk, which requires a fine structure in the order of nm and that deformation of signals may occur due to protrusion of the fillers from the surface of the substrate or expansion of the fillers.
A plastic substrate is liable to be deformed by heat and moisture caused during the manufacturing process, a user's service environment, and a retention environment on the basis of the above-described various factors, and in the present circumstances, there is no single molding material for a disk, which is optically transparent and withstands moisture absorption, and deformation due to residual stress and contraction deformation of a protective film caused upon film formation.
Along with a tendency toward high density recording, particularly, toward high numerical aperture (NA), there arises a problem in reducing allowable values of warping and waviness, failing to readout recorded signals from a disk or to record signals on the disk, and to solve such a problem, it is expected to develop a disk with less deformation and waviness irrespective of changes in manufacturing process and service environment.